Any trained bovine is called an ox. Almost always a steer, because bulls are too difficult, and cows are too valuable in other ways. Every ox I'm aware of is a steer. We start them usually as bottle fed babies, really cute, and then they keep in training as they grow up.
Most are from dual purpose breeds, or dairy breeds, because the beef breeds get too heavy and have trouble actually working. The biggest team I ever saw was a team or Aryshires, and with their upturned horns, really impressive!
Mary